Global warming might cause … colder winters, more snow

Averaged globally, temperatures around the world have been rising pretty much since modern instrumental record-keeping began. But average global warming doesn’t mean steady, year-round warming everywhere.

Led by Judah L. Cohen of Atmospheric and Environmental Research, the new study explains the effect this way:

The Arctic, which is warming more strongly than other parts of the globe, now sees more sea ice melt all the way into late summer and early fall;

The warmer conditions over open Arctic waters (remember, dark waters absorb heat from sunlight while light-colored ice reflects it) lead to higher moisture levels in the air. More moisture means more clouds that can bring on more precipitation;

Over Siberia, where it’s still cold enough for snow instead of rain, those additional clouds and moisture mean more snow than normal, especially in the fall;

Increased autumn snow cover in Siberia can help bring on the winter phase of what’s called the Arctic Oscillation, a variation in surface atmospheric pressure patterns. That can affect the jet stream and other prevailing wind patterns, bringing blasts of Arctic air deep into lower latitudes, especially in the eastern United States, southern Canada and northern Eurasia.

That line of reasoning would help explain why the Northern Hemisphere has seen a trend of more extreme winter weather — both bitter cold spells and heavy snows — since around 1988, the researchers say.

“(E)vidence suggests that summer and autumn warming trends are concurrent with increases in high-latitude moisture and an increase in Eurasian snow cover,” they write in an article published in Environmental Research Letters. And that, they continue, “dynamically induces large-scale wintertime cooling.”